1. Field of the Invention
This invention relates generally to radiation imaging systems and in particular to high voltage x-ray radiography imaging systems.
2. Discussion of the Prior Art
X-ray computed tomography (CT) has been used for the non-destructive determination of the interior features of objects. While typical medical computed tomography systems operate at or below a peak voltage of 120 kilovolts, computed tomography systems for industrial uses often operate at a peak voltage of 400 kilovolts or greater. As used herein, "peak voltage" or the like refers to the maximum energy of the accelerated electrons producing the x-rays. Such high voltages are required for the x-rays to penetrate large and high density objects. It is desirable for the x-ray detectors used in these high voltage industrial applications to absorb high energy x-rays and to provide high resolution for the measurement of wall thicknesses, distortions, and defects.
One type of detector for high voltage x-rays utilizes modules of photosensitive diode arrays. Each module contains sixteen single crystal diodes with the diodes arranged parallel to one another. The spacing between each diode is 1 mm. The diode modules are oriented with the diode edges normal to the incident x-ray. Each module is then focally aligned with the x-ray source. Lumex, a solid state scintillator without light guiding properties, is commonly used for converting incident x-rays to visible light. This detector configuration, using modules of parallel edge-on photosensitive diodes, is limited to a resolution of 0.5-1.0 lp/mm, because of limitations of the scintillator and diode technology.
A second type of x-ray detector is a xenon (Xe) detector. Charge is generated by the absorption of x-rays in highly pressurized xenon gas. The generated charge is collected by a high voltage applied to an electrode, and an output signal is read out of patterned electrodes. Since this xenon detector experiences electron and x-ray scattering within the relatively low density xenon gas, the detector is limited to a resolution of about 2.8 lp/mm.
None of the aforementioned x-ray detectors has sufficiently high resolution to produce a desired attenuation map of the interior of a solid object.